It seems that AuthManager checks the user rights too stringently (when calling isBlockedFromCreateAccount()). In addition to checking whether the account creator can create the new account (AuthManager::checkAccountCreatePermissions()), it also checks whether the new account can create itself (CheckBlocksSecondaryAuthenticationProvider::testUserForCreation()). I think that is incorrect, but perhaps there is a reason for this (or perhaps it has always been this way, and you're in fact requesting a new feature).

Today, I was not able to reproduce it on https://test.wikipedia.org (1.33.0-wmf.20 (f929e2a) 23:04, 6 March 2019). I tried a non-anonymous block of an IP range and both an anonymous and a non-anonymous block for a single IP.